1. Field of the Invention
The present invention relates to radio frequency polarisation, particularly microwave polarisation, and to communication systems utilising signals of a defined polarisation.
2. Description of Related Art
Satellite communications normally use circularly polarised signals. This is to economise on bandwidth by frequency re-use, where right-handed circular polarisation is used on the up-link and left-handed on the down-link. In addition, the source and receive antennas may be oriented by any angle with respect to each other without a significant loss of signal.
A polariser placed between the antenna feed and the rest of the system converts linearly (i.e., plane) polarised transmitted signals into right-handed circular polarisation, and converts received left-handed circular into the orthogonal linear polarisation. An orthomode transducer is then used to separate these two linear polarisations that, in normal operation, are simultaneously present in the waveguide behind the polariser.
Such communication systems may employ either a splashplate or a polyrod as an antenna feed. A splash-plate comprises a rod of dielectric material which extends from a tubular metal waveguide (generally air-filled) and expands into a generally conical portion. The base of the conical portion is generally convex and is covered with a metal film, which film acts as a subreflector. A polyrod simply comprises a rod of dielectric material which extends from a tubular metal waveguide (generally air-filled) towards a conventional dish antenna. In either case the impedance of the dielectric rod has to be matched to that of the tubular metal waveguide, and this is achieved by conically tapering the dielectric rod (which is invariably of circular cross-section) to a point. The longer the tapered portion, the better the impedance matching. In practice, in view of the limited space available, the taper is made about two wavelengths long (corresponding to a length of 100 mm at X-band), which gives acceptable matching only over a bandwidth of around 15%.
In addition to the limitations imposed by the impedance-matching taper, the size of the system is increased and/or its performance is compromised by the characteristics of the polariser. A variety of microwave polarisers are known for use in tubular waveguide, and generally consist of sets of slots in the waveguide walls or bolts inserted through the slots in the waveguide walls or bolts inserted through the waveguide and oriented in an appropriate manner to differentially phase-shift the microwave radiation to achieve the required polarisation. One other type of microwave polariser, namely the vane polariser, consists of a thin sheet of dielectric material cut into two identical isosceles triangles, which triangles are joined at their apices to form a symmetrical coplanar "bow tie" which is located in an axial plane of the waveguide with the bases of the triangles perpendicular to the waveguide axis. A component of microwave radiation propagating axially in the plane of the "bow tie" experiences a greater mean dielectric constant than a component (which is essentially unaffected) propagating axially in a plane perpendicular to the "bow-tie" and accordingly undergoes a differential phase shift. The tapering edges of the triangles provide the required impedance matching, and the vane polariser necessarily has an appreciable length (typically two guide wavelengths).
One further example of a polariser is known from U.S. Pat. No. 3,216,017 in which a wedge formation is used to achieve polarisation. It is, however, essential to this prior art that the polariser be part of a waveguide transition from rectangular waveguide to circular waveguide. The rectangular guide limits the use of the polariser to conversion between a single linearly polarised wave and a circular or elliptical wave whereas the present invention is concerned with accommodating simultaneous orthogonal linearly polarised signals of the same frequency. Again, the rectangular/circular transition is essential to obtaining an impedance match in this prior art reference since the axial position of the dielectric wedge within the transition is adjustable in relation to the transition to obtain a match. The present invention is concerned with providing a polariser for both polyrod feeds and splashplate feeds and in the latter case axial movement of the dielectric and splashplate is not permissible since this would involve movement of the sub-reflector relative to the main reflector. Matching in the present invention is provided, as will be seen, by other means.